I had the same problem on my 2020 RS7, Audi want £2500 to strip interior and dry carpets with finding the leak as an extra cost. Managed to remove fasteners from the edge of the undertray and removed the front and rear bungs from both chassis rails. Water poured out but not sure how the water is getting in there. Left the bungs out and no more problems so far.
I have an after market warranty which, of course, does not cover water ingress....